In certain computer control applications, it is necessary to track and measure an image of an object passively. It is especially important in weapons delivery systems that a target be so tracked. If such a target were tracked actively, (i.e., using radar or laser range finding techniques) the target might detect the presence of the tracker or tracking system. Once the target has detected the presence of the tracker, it can respond in one of several ways, all of which are deleterious to the tracker. For instance, the target might "jam" the tracker by bombarding it with signals that are comparable to those which the tracker is actively using. The target might also fire its own weapon at the tracker, at the source of the tracking signal, or, even at the launching site of the tracker. In this way, the target could defeat the tracker, destroy the tracker or perhaps even destroy the launch site of the tracker, including the operating personnel.
Passively tracking a target, however, imposes at least one serious limitation on the tracker. A tracker cannot accurately determine the distance or "range" to a target if it cannot actively sense the object. An active tracker, for instance, could determine the distance to a target by measuring the elapsed time from the emission of a radio frequency signal to the receipt of the signal reflected off of the target. The absence of a range measurement from tracker to target limits the passive tracker's ability to compensate for the apparent change in target image as the tracker moves in relationship to the target. Without this ability, a tracker will fail to maintain a constant target.
In practice, a tracker benefits by tracking several subimages of its target's image. These subimages are two dimensional representations of structures that are physically connected to the exact target location or "aimpoint" in the real three-dimensional world. Multiple subimages are used for redundancy purposes and because the actual aimpoint of the target is often untrackable. As the tracker nears the target, however, the subimages will appear to move with respect to each other. The position of the subimages with respect to one another may also change in certain situations. For instance, two subimages located on a target may appear to approach one another if they are located on a face of a target that is rotating away from the tracker. A tracker targeting an elongate structure such as a runway or tall building will sense complex subimage motion due to closure of the tracker on the target. These subimages will appear to move at rates that are dependent on their location within the tracker's field of view.
Prior attempts to passively track an object have resulted in solutions with limited flexibility and poor accuracy. Heretofore, an object once identified as an aimpoint was tracked by tracking a predetermined number of subimages in a known pattern. Typically, the pattern chosen was a square with the aimpoint at its center and four subimages located at the four corners of the square. That system would track the four subimages located at the corners of the square and infer he actual aimpoint using the simple symmetry of the predetermined square. This method faltered when the geometry of the actual target resulted in less than four suitable subimages located in the requisite pattern. This system also lacked the ability to use trackable subimages that were not in the requisite pattern.
Therefore, a need has arisen for a passive subimage tracker which is able to track any number of subimages arbitrarily related to an aimpoint without range data.